Mucus clearance, a critical mechanism for airway hygiene, relies on the biophysical properties of the mucus gel. In COPD, quantitative and qualitative changes in mucus contribute to airway obstruction, exacerbation rate, accelerated lung function decline and mortality. This COPD subphenotype, also known as chronic bronchitis (CB), affects up to 70% of patients with the disorder. Despite the prevalence, complexity and importance of mucus hypersecretion in COPD, our understanding of the qualitative changes in mucus that accompany COPD development and progression is quite limited. Given that the CB subphenotype of COPD, referred to as COPD-CB, is intimately linked to adverse outcomes, a better understanding of the signature pathology in this disorder may lead to more effective targeted therapies and more practical disease classifications. We have pioneered the use of high resolution multiple particle tracking of muco-inert nanoparticles, termed particle tracking-based structural analysis (PTSA), to probe mucus microstructure. Using this approach in addition to mucus content measurements, we have found that distinct properties of spontaneously expectorated sputum obtained from COPD patients associate with disease severity. We also have found that elevated solid content increases osmotic pressure in COPD sputum samples, which in turn impairs mucus clearance by collapsing periciliary layer above the airway epithelium. One of the stated objectives of SPIROMICS is a more refined characterization of COPD subphenotypes. We submit that a detailed assessment of the properties of spontaneous sputum samples obtained from a large, well-defined, study population of COPD-CB patients in the SPIROMICS cohort will avail informative biophysical and biochemical measures, which associate with disease severity and progression. Our hypotheses are 1) altered mucus microstructure, specifically, a tightening of the mucus mesh, and increased osmotic pressure associates with worsened lung function, accelerated lung function decline, increased symptoms, and worse quality of life in the COPD-CB population 2) frequent exacerbators will display stereotyped mucus alterations in both acute and quiescent settings. We plan to collect spontaneous expectorated sputum samples from 150 SPIROMICS patients who report cough and chronic sputum production at four clinical sites/subsites and assess osmotic pressure and microstructure using the innovative PTSA methodology, in addition to conventional biochemical analyses of sputum constituents.